1) All the cards that has the attribute Duration have explicitly defined Duration. This means that it is explicitly defined how many turns they will be active (their other attributes will be involved in the combat).
2) All the cards that DOESN'T have the attribute Duration have default Duration.
3) For the buff cards the default duration is infinity. This means that the buff cards with no attribute Duration will last until the end of the combat IF they're not removed by you during an Equip Phase or by the enemy, using the attribute Decrease opponent's buff area.
4) For all non-buff cards, which means all the cards that can be played during Attack/Defence Phase in the play area (that includes the Hero Special Card), have default Duration = 1. This means that they will be active only in the current turn. Also the attributes Increase Duration and Decrease your opponent's Duration will not affect these cards.
5) The attribute "Increase Duration" affect only the active or currently played non-buff cards with specified Duration (i.e. with explicitly defined Duration >1 - see example 1)
6) The attribute "Decrease your opponent's Duration" affect only the active or currently played non-buff cards with specified Duration of your opponent (i.e. with explicitly defined Duration >1 - see example 1)
7) The attribute "Collect all cards" affect only the active or currently played non-buff cards with specified Duration of both you and your opponent (i.e. with explicitly defined Duration >1), bringing the Duration of all active cards on the table to zero (they will be removed for the next turn).
8) There are some skills that boost the attributes "Increase Duration" and "Decrease your opponent's Duration".
9) There is no Duration cap for any card. You can increase the duration of a card with specified Duration to values above its natural Duration (see example 2).

Example 1:
In turn 5 you play a card A1 with attributes Bonus Damage = 12 and Duration = 2 and a card A2 with attributes Damage = 4 and Duration = 3. The opponent has 30 HP left. He plays a card B1 with Defence = 10 (no attribute Duration, which means that his card B1 has default duration = 1 and will be active only in the current turn). First the damage calculations occur. In this case you will deal 4+12-10=6 damage to your opponent and at the end of the turn he will have 30-6=24 HP.
Starts the next turn 6. The card A1 has Duratuon = 2, so it was active in the previous turn 5 (when it was played) and will be active in the next turn (the current turn 6). So the card A1 is still active with left Duraton = 1 (in the bottom edge of the active cards you can read all this information for each active card - in this case you will read 1/2, which means that there is 1 turn in which this card A1 will be active before it expires from total 2 turns) and the card A2 is also active with left Duration = 2 (in the bottom edge you will read for card A2 2/3). Here you decide to push the enemy and play a card A3 with attribute Increase Duration = 2 and a card A4 with Bonus Damage = 15. Your opponent plays a card B2 with Defence = 20. Now the damage calculations start: you will deal to your opponent 12(card A1) + 4 (card A2) + 15 (card A4) - 20 (card B2) = 11 damage. So he will end the turn with 24-11=13 HP. After all the damage calculations and all the healing calculations start the Duration calculations. In this case the duration of card A1 should go to zero, but because of the card A3 (with Duration = 2) it gets 0+2=2 Duration more. So in the next turn 7 it will be with duration 2/2. Card A2 should go to 1/3, but because of the card A3 it gets 1+2=3 Duration more. So in the next turn 7 it will be with duration 3/3. The card A4 has default unspecified Duration (of 1 turn) so it is not affected by the card A3 and it's Increase Duration attribute.
Starts the next turn 7. You have the card A1 active (2/2) and card A2 active (3/3) and both of them will deal 4+12 = 16 damage. And your opponent has only 13 HP left. You decide here to finish him, making him unable to defend. So you play just one card A5 with the attribute Reduce Opponent's Defence = 20. But your opponent is in Defend and knows that he must survive your attack, so he plays a card B3 with attribute Reduce Opponent's Damage = 8 (unaffected by the attribute Reduce Opponent's Defence of your card A5) and a second card B4 with the attribute Decrease your opponent's Duration = 1. First the damage calculations start - you will deal to your opponent 4+12-8 = 8 damage (your reduce defence doesn't affect anything). Your opponent now has 13-8 = 5 HP left. And now the Duration calculations occure - your card A1 should have left duration 1/2, but because of your opponent's card with attribute Decrease your opponent's Duration = 1, A1 will come to 0/2 and will vanish from the table. And the card A2 instead of 2/3 will appear in the next turn with left duration 1/3.
CONCLUSION from Example 1: If you had low HP, for example 20 HP, and your opponent has very strong attacking cards, in turn 8 he will be in Attack Phase, and even with 5 HP he can still kill you and take the victory. This is an example how the Increased Duration and the Decreased Duration can make big strategic difference in the game.

Example 2:
In turn 5 you play a card A with Duration 3 and a second card B with attribute "Increase Duration" = 2. In the next turn 6, the card A instead of Duration 2/3 (two turns left before expiration from total 3 turns natural Duration) will have Duration 4/3, because of the effect of card B. This means that the new duration of the card A will be 4 turns.